Nasal continuous positive airway pressure (CPAP) is the nonsurgical treatment of choice for obstructive sleep apnea syndrome (OSAS). Its efficacy for eliminating apneas and hypopneas is well established, but there continue to be significant questions about the consequences for daytime functioning of regular vs. irregular CPAP use. The project addresses these clinical issues, building on results from our two recent studies. In the first, a microprocessor-monitor inside the CPAP machines of 35 OSAS patients revealed that only 46% of them actually used CPAP regularly (>=4 hrs/night on >=70% of nights). Compared to irregular CPAP users, regular users were more likely to report post-treatment improvements is daytime energy, suggesting that CPAP use may be associated with the magnitude of improvemnet in hypersomnolence experienced by patients. Protocol I is designed to test the hypothesis that regular CPAP use results in significant improvements in objective measures of daytime sleepiness relative to irregular use. However, we do not expect regular CPAP use to yield daytime functioning comparable to that of age and sex- matched controls, because in a second study we found that CPAP significantly improved objective and subjective measures of daytime sleepiness, but the level of functioning appeared to be suboptimal, and as soon as CPAP was withdrawn, hypersomnolence returned to pre-treatment levels. Therefore, Protocol II will determine whether the residual sleepiness present when patients are using CPAP regularly vs. irregularly is the result of an inadequate sleep duration. Because we believe that most of the residual sleepiness of regular CPAP users is due to inadequate sleep durations, we hypothesize that they will be able to extend their sleep on CPAP and this will eliminate their residual daytime sleepiness relative to that after a typical night of CPAP use. On the other hand, we expect that sleep extension with CPAP use will have less of an effect on the hypersomnolence of irregular users. This prediction is based on our view that irregular users of CPAP are a heterogeneous group, made up of patients who cannot extend sleep on CPAP, as well as those who cannot derive benefits from doing so. Therefore a subgoal of Protocol II is to determine the proportion of irregular users who do not benefit from extended sleep on CPAP. Thus, we seek to answer basic clinical questions regarding the effectiveness of CPAP for relieving hypersomnolence, relative to the regularity of its use and the duration of sleep when it is used. Such answers should improve our ability to effectively treat OSAS patients. We also seek to continue to refine the techniques we have developed for assessment of CPAP use, ambulatory motility, daytime vigilance, mood, and functional status, which will be needed for future studies on the epidemiology and natural history of OSAS and its treatment.